1. Field of the Invention
The present invention relates to an intelligent welding gun provided with a sensor, and various kinds of methods conducted by using the welding gun including a calibration method of a sensor, a control method of welding (which includes, for example, a control method of a pressing force, a feedback control method of re-welding, a control method of welding strength, a control method of suppressing the generation of a weld expulsion, and a control method of a track of a welding robot), and a managing method of a change in a positional accuracy of a welding point.
2. Description of Related Art
Japanese Patent Publication No. 10-94882 discloses a method for controlling a pressing force of a welding gun. In the method, a small quantity of an elastic displacement, generated in a fixed side electrode tip when a moving side electrode tip is driven to contact the fixed side electrode tip and further driven to press the fixed side electrode tip, is detected by an encoder of a servo motor for driving the moving side electrode tip. More particularly, the elastic displacement is determined based on the rotation increment of the servo motor generated from the time when the moving side electrode tip begins to contact the fixed side electrode tip to when the electric current suddenly increases when the moving side electrode tip is further driven to press the fixed side electrode tip. A real pressing force between the electrode tips is calculated based on the measured elastic displacement of the fixed side electrode tip. Then, a setting pressing force between the electrode tips is modified to be equal to the calculated real pressing force.
However, there are the following problems with the above-described conventional method for controlling a welding gun.
First, since a moving side portion of the welding gun including the moving side electrode tip and the servo motor for driving the moving side electrode tip is provided with a speed reducer, a mechanical impedance of the moving side portion is greater and dynamically harder than that of a fixed side portion of the welding gun. In this instance, the mechanical impedance is defined as an impedance expressed by a vector of .vertline.m, c, k.vertline., when a movement of the electrode tip is expressed by an equation: m.multidot.d.sup.2 x/dt.sup.2 +c.multidot.dx/dt+kx=F (t). In a case where the vector only includes k, the mechanical impedance is a spring constant. The encoder is located on the opposite side of the moving side electrode tip with respect to the speed reducer of the servo motor, so that a pressing quantity of the moving side electrode tip, transmission of a change in the pressing displacement and the pressing force to the encoder through the speed reducer is a small amount and is delayed. As a result, the responsibility is lowered, so that it is difficult to obtain an accurate response with the conventional method, in which the welding gun is controlled based on an output of the encoder.
Second, since the rigidity of an arm supporting the fixed side electrode tip is increased so that the mechanical impedance of the fixed side portion including the fixed side electrode tip and the arm is nearly equal to the mechanical impedance of the moving side portion, the size and the weight of the arm is large, which causes the welding gun to be increased both in size and weight.